1. Field of the Invention
The present invention relates generally to segment reallocation, and more particularly, to an apparatus and method for reallocating segments in a broadband wireless communication system.
2. Description of the Related Art
An Institute of Electrical and Electronics Engineers (IEEE) 802.16e system employs a cellular scheme and supports a frequency reuse factor of 1. As a result, neighboring cells can use the same frequency. Thus, a Mobile Station (MS) existing in such a system has to be able to distinguish a Base Station (BS) where the MS resides, from a neighboring BS among a plurality of BSs, which use the same frequency. For this purpose, whenever a frame is transmitted to the MS, each BS transmits the frame by inserting a Pseudo Noise (PN) code to a preamble, which is a first symbol of the frame.
According to an IEEE 802.16e standard, a total of 114 preamble PN codes are defined, and the codes have code indices 0 to 113, respectively. In addition, the preamble PN codes each have an Identification (ID) cell (hereinafter, referred to as IDcell) and a segment number. By analyzing the preamble PN codes, the MS can recognize a code index, an IDcell, and a segment number of a corresponding BS. The IDcell has 32 values (i.e., 0 to 31). The segment number has three values (i.e., 0 to 2). Not all codes have unique IDcell and segment number combinations. Thus, among the 114 codes, only the codes 0 to 95 have unique IDcell and segment number combinations. The IDcell and segment number combination of the codes 96 to 113 are duplicated with that of the codes 0 to 95.
A segment is used for various purposes, and a result of segment allocation has a significant effect on system performance. The segment determines a carrier set through which a preamble is transmitted. The preamble is transmitted through only a ⅓ part of a sub-carrier, which remains after removing a guard band. The sub-carrier set may be determined by using Equation (1) below.carrier set=segment+3k (k=0, 1, 2, . . . )  Equation (1)
Equation (1) shows that, when the same segment is allocated to neighboring sectors, preambles of two sectors are transmitted through the same carrier set. In this case, even if an MS can obtain a preamble without any problems because different code indices are allocated to the preambles of the two sectors, downlink throughput may deteriorate.
In a downlink channel, the MS estimates a pilot signal transmitted from a BS. The estimation result is used in a demodulation process. However, similar to a Frame Control Header (FCH) and a Downlink-MAP (DL-MAP), when information is transmitted during first few symbols of a frame, no pilot signal is transmitted from the BS, resulting in difficulty in channel estimation. Therefore, the MS estimates a channel by using the preamble. In this case, whether a carrier set is duplicated between neighboring sectors significantly affects capability of channel estimation. If different segments are allocated to neighboring sectors and thus preambles are transmitted through different carrier sets, then channel estimation can be accurately achieved through the preamble, which leads to improved modulation capability of the FCH and the DL-MAP. On the other hand, if the same segment is allocated to the neighboring sectors, channel estimation is inaccurately achieved through the preamble, which may result in deterioration in downlink demodulation capability.
Moreover, a segment may determine a frequency band used in a downlink Partial Usage of Sub-Carrier (PUSC) zone. In the downlink PUSC zone, a sub-carrier is divided into 6 groups, which include 3 major groups and 3 minor groups. Each sector may use all or some of the 6 groups, and information thereof is transmitted to the MS through the FCH. In this case, each sector must use one or more major groups, and the number of major groups to be used is determined by a segment allocated to the sector. In order to reduce interference between cells in a cell boundary area, the system may allow some groups to be used between sectors, and this will be referred to as segmented PUSC. If the neighboring sectors use the same segment, the two sectors use the same group. As a result, interference is not diminished even when the segmented PUSC is used, which leads to deterioration in downlink throughput.
As such, when the same segment is allocated to neighboring sectors, downlink modulation capability may deteriorate. Therefore, segment allocation may significantly affect system performance. However, since the number of segments is limited to three, it is not easy to allocate segments while minimizing segment duplication between the neighboring sectors.
Even when segments are effectively allocated, the segments may have to be reallocated later in some cases. For example, existing segment allocation may no longer be useful when there are changes in radio configurations such as locations of some BSs within a system, transmission power, an antenna angle, an antenna type, etc. Furthermore, the existing segment allocation may not be preferable when a radio-wave environment changes due to alternation of buildings and topographies. If this is the case, segment reallocation is needed. The segment reallocation may be carried out by allocating new segments to all sectors in the system according to a conventional algorithm. However, when changes are significant in new segment allocation, this may affect operations of the system.
Accordingly, there is a need for a segment reallocation method in which, if required, segments are reallocated by using a result of existing segment allocation, so as to improve segment allocation capability while minimizing changes in segment allocation.